Russian officials initially called the entire attack a hoax meant to provoke Western support for Syrian rebels.

As mounting evidence made that position harder to defend, Russia's military instead claimed the attack happened at midday and was targeting a terrorist weapon stockpile that contained chemical weapons.

Although the official U.S. statement stops short of saying Russia knew about the attack in advance, at least one report says Moscow was aware the chemical attack was to take place, then afterward bombed a hospital to destroy evidence.

Secretary of State Rex Tillerson visited Russia on Tuesday after meeting with leaders of the G-7 nations in Italy.

The countries won't impose new sanctions on Russia for now, but Tillerson said Russia's influence in the Middle East is at risk now that President Bashar al-Assad's hold on Syria is "coming to an end."